650 



THE POPULAR SCIENCE MONTHLY 



vations establish beyond a doubt that the motion of a jet of 

 water is never continuous, but is intermittent, periodic. Design 

 vii, Fig. 5, which represents a jet from a hydrant or fire-engine 

 at the moment of elevating, and design viii, representing a ver- 

 tical jet from a narrow hole, are schematic but faithful reproduc- 

 tions of numerous observations. The photographs of Figs. 6 and 

 7, taken from nature at the overflow of a factory flume on Sun- 

 day, when no wheel or turbine was moving, are convincing. 



Fig. 6. Vibratory Motion of Falling Water. 



Fig. 7. Vibratory Motion of Falling Water. 



This intermittent character of the stream is due to the same 

 cause as the sound produced by the air in blowing through a key- 

 hole ; it is produced in the same way as the wavy tufts in smoke 

 escaping from a chimney. Water, as well as air, is elastic, but in a 

 less degree, and differences in its pressure are propagated in waves. 

 Ample explanations on the subject may be found in the chapters 

 on undulations in treatises on physics; but the theory of the 

 liquid wave is complicated, and far from being exhausted. We 

 limit ourselves to saying that water, like air, is greatly disposed, 

 every time the velocity of the stream is changed, by shock against 

 a foreign body or by an abrupt contraction or enlargement of the 

 channel, to go into vibrations and to communicate them to the 

 walls of the orifice whence it escapes or to any object against which 



